Indian Journal of Chemistry
Vol. 47A, October 2008, pp. 1524-1527
Effect of inorganic ions on surface properties
of a non-ionic surfactant at various
temperatures in aqueous medium
Ram Partapa,* & O P Yadavb
aDepartment
of Chemistry, Government College,
Hisar 125 001, India
bDepartment of Chemistry,
CCS Haryana Agricultural University Hisar 125004, India
Received 14 December 2007; revised 6 June 2008
Studies on surface tension of iso-octyl phenoxy polyethoxy
ethanol in water and in the presence of aqueous Na 2SO4 and
Na3PO4 salts having varying anion valency at 288, 293 and 298
K are reported. The values of critical micelle concentration,
maximum surface excess concentration and minimum area per
molecule of the surfactant at air-liquid interface have been
evaluated. Thermodynamic parameters of micellization and
those of adsorption have also been evaluated. The micellization
process appears to be endothermic in nature while negative
values of G0m and positive values of S0m support its
spontaneity.
The solution characterisation and intermolecular
interactions of surface active substances in aqueous
medium are supposed to be landmark in the fields of
interaction of medicinal solutions, agrochemicals,
detergency, solubilizing power, enhanced oil recovery
and in metallurgical process1-4. There have been
several reports5-7 on physico-chemical properties such
as surface tension, specific conductance, viscosity,
fluorescence, dye solubilization, etc. on ionic as well
as non-ionic surfactants in aqueous solutions. Though
a number of reports have appeared on the surface and
thermodynamic properties of surfactants in aqueous
electrolyte solutions at temperatures 298 K or above,
yet the data for such systems at low temperatures are
scanty8-13. We report herein the data for critical
micelle concentration (CMC), surface pressure at
CMC (cmc), surface excess concentration (max),
minimum area per molecule at the air-liquid interface
(Amin), and thermodynamic parameters of micellezation/adsorption for iso-octyl phenoxy polyethoxyethanol (TX-100) in water and in aqueous sodium
sulphate (Na2SO4) and sodium phosphate (Na3PO4) at
288, 293 and 298 K.
Experimental
TX-100 (SD Fine, purity 98%), Na2SO4 and
Na3PO4 (BDH, purity 99.9%) used were of AR grade.
The solutions were prepared in doubly distilled water
having a specific conductance of 2.00×10-6 -1cm-1 at
298.15 K.
Surface tension of solutions was determined by
dropweight method using a specially designed
stalagmometer described elsewhere14. The stalagmometer was calibrated by determining the surface
tension of pure liquids: benzene, carbon tetrachloride,
n-hexane, acetophenone and water as standard.
Reproducibility of results were within +0.2%
literature values. The measurements were made in a
thermostat (Tempstar, model KW201A) which
provided temperature control within +0.01 K.
Results and discussion
From the plots of surface tension versus log
[surfactant] for the studied systems, the CMC values
have been obtained from the sharp break points. CMC
values are recordrd in Table 1. The observed CMC of
pure TX-100 solution at 298 K agrees well with the
reported literature values15,16. It is clear from Table 1
that CMC of pure TX-100 decreases with increase in
temperature. In case of non-ionic surfactants in the
absence of any additive, the depression in CMC may
due to the dehydration of the hydrophobic moiety of
the surfactant molecules and also due to breaking of
water structure17,18 with increasing temperature which
facilitates micellization. The process of micelle
formation in non-ionic surfactants is controlled by
hydrophobic interaction as well as London dispersion
force.
Table 1 shows that decrease of CMC of the
surfactant with the addition of Na3PO4 is more than
Na2SO4. The effect of addition of Na2SO4 and Na3PO4
on the CMC value of TX-100 may be partly due to the
salting out of the hydrated ethylene oxide condensate
of the surfactant and partly due to ion-dipole
interaction (in the Stern layer) of Na+ and negative
dipole of the hydroxyl group of the surfactant19,20.
Maximum surface excess concentration (max)
values at the air-liquid interface has been obtained
using Gibb’s adsorption equation9:
max = – 1/2.303 nRT (d/d log C)T
… (1)
NOTES
1525
Table 1 — Critical micelle concentration (CMC), surface excess concentration (max), minimum area per molecule (Amin) and
surface pressure at CMC (cmc) for Triton X-100 +electrolyte systems
[Na3PO4]
(mol dm-3)
Temp.
(K)
CMC103
(mol dm-3)
max1010
(mol cm-2)
Amin102
(nm2)
cmc
(mN m-1)
2.18
2.09
2.01
1.85
1.69
1.58
1.81
1.60
1.50
1.63
1.52
1.40
72.6
79.4
82.6
89.8
98.2
105.1
91.7
103.8
110.7
101.9
109.2
118.2
35.8
36.9
38.1
41.2
41.4
41.8
42.2
42.5
42.8
42.6
43.2
43.2
1.81
1.66
1.55
1.65
1.52
1.40
1.56
1.43
1.31
91.7
100.0
107.1
100.6
109.2
118.6
106.4
116.1
126.7
43.3
43.7
44.2
44.4
44.6
44.8
45.4
45.6
45.9
Na2SO4 + H2O
0.00
0.025
0.050
0.075
288
293
298
288
293
298
288
293
298
288
293
298
0.49 (0.52)
0.42 (0.44)
0.36 (0.38)
0.42 (0.42)
0.35 (0.38)
0.28 (0.34)
0.37 (0.38)
0.30 (0.32)
0.22 (0.28)
0.31(0.34)
0.25 (0.28)
0.19(0.22)
Na3PO4 + H2O
0.025
0.050
0.075
288
293
298
288
293
298
288
293
298
0.40 (0.39)
0.32 (0.34)
0.25 (0.30)
0.35 (0.34)
0.27 (0.30)
0.19 (0.28)
0.28 (0.30)
0.21 (0.26)
0.14 (0.24)
CMC values given in parenthesis have been obtained from the viscosity method
where ‘n’ is the number of particles released per
surfactant molecule in the solution and R, the gas
constant. (d/d log C)T represents the slope of the
surface tension versus log C plot below the CMC at
constant temperature T. In the present investigation
n=1 for non-ionic surfactant. The calculated values
for max for the studied systems at three temperatures
are also presented in Table 1. It may be seen from
Table 1 that the max values decrease with the increase
in temperature which may be due to the enhanced
molecular thermal agitation at higher temperature.
These results are in conformity with results reported
elsewhere21. A further decrease in max values in the
presence of aqueous Na2SO4 and Na3PO4 may be due
to the fact that addition of these electrolytes causes a
partial displacement of surfactant molecules from the
air-liquid interface to the bulk phase.
The minimum area per molecule (Amin) at the
liquid-air interface has been calculated using the
relation:
Amin = 1014/N max
… (2)
where ‘N’ is Avogadro's number. Amin values for the
studied systems are given in the Table 1. An
examination of these values reveals that Amin increases
both with the increase in temperature as well as with
the concentration of Na2SO4 and Na3PO4 in the
surfactant solution. This behaviour can be explained
in terms of the enhanced compatibility of surfactant
with the solvent in the presence of electrolytes,
thereby, causing a shift of surfactant molecules from
air-liquid interface to the bulk phase.
Surface pressure at CMC (cmc), an index of
surface tension reduction at CMC, has been calculated
using the equation9:
cmc = 0 – cmc
… (3)
where 0 = surface tension of water and cmc = surface
tension of surfactant solution at CMC. cmc values
(Table 1) show marginal increase with increase in
temperature.
INDIAN J CHEM, SEC A, OCTOBER 2008
1526
The thermodynamic parameters like standard
Gibb’s energy change of micellisation (G0m),
standard enthalpy change (H0m) and standard
entropy change (S0m) (Eqs 4-6) have been calculated
using the equations9, respectively:
G0m =RT ln X
… (4)
(where X is the surfactant mole fraction at CMC)
S0m = [ –d(G0m )/ dT]P
… (5)
H m =G m + TS
… (6)
0
0
0
m
The various thermodynamic parameters of
micellization calculated using Eqs (4)-(6) are
presented in Table 2. The G0m values are found to be
negative indicating the spontaneity of micellization
process in aqueous system. The S0m values are
positive for the studied systems. It may be due to
breaking of water structure when the surfactant
hydrophobic chain transfers from bulk water to the
micellar core. There is a further increase in S0m on
adding Na2SO4 and Na3PO4 to the surfactant solution
which may be partly due to water structure breaking
effect of Na2SO4 and Na3PO4 and partly due to the
desolvation of prehydrated9,22 ethylene oxide chains
of surfactant molecules in the presence of Na2SO4
and Na3PO4.
The standard enthalpy of micellization H0m is
positive due to the hydrophobic-hydrophobic
interaction of surfactant alkyl chain in the process of
micellization of the non-ionic surfactant (TX-100).
Thermodynamic parameters of adsorption, viz.,
G0a, H0a and S0a have been calculated using the
following relations23 at constant pressure:
G0a = G0m – 6.023  10 –1 cmc . Amin
… (7)
S0a = – d(G0a) / dT
… (8)
H0a = G0a + T S0a
… (9)
The values of G0a, H0a and S0a are presented in
Table 2. The lower G0a values compared to G0m
(Table 2) indicate that adsorption of the surfactant
molecules at the air-liquid interface is preferred over
the micellization. The S0a values are all positive and
larger than S0m. This may be due to more degree of
Table 2 — Thermodynamic parameters of the micellization/adsorption for Triton X-100 system
[Na2SO4]
(mol dm-3)
Temp.
(K)
–G0m /–G0ad
(kJ mol-1)
H0m / H0ad
(kJ mol-1)
S0m / S0ad
(kJ mol-1K-1)
21.1 / 28.1
0.17 / 0.20
28.8/ 38.4
0.198 / 0.239
37.3/ 49.3
0.228 / 0.278
35.1 / 50.7
0.222 / 0.285
33.6 / 44. 5
0.215 / 0.261
43.7 / 55.7
0.251 / 0.302
49.6 / 62.8
0.273 / 0.329
Na2SO4 + H2O
0.00
0.025
0.050
0.075
288
293
298
288
293
298
288
293
298
288
293
298
27.9 / 29.5
28.7 / 30.5
29.6 / 31.5
28.3 / 30.5
29.2 / 31.6
30.2 / 32.8
28. 6 / 30.9
29. 6 / 32.2
30.8 / 33. 7
28.9 / 31.5
30.0 / 32.8
31.2 / 34.4
Na3PO4 + H2O
0.025
0.050
0.075
288
293
298
288
293
298
288
293
298
28.4 / 30.8
29.4 / 32.0
30.5 / 33.4
28.7 / 31.4
29.8 / 32.8
31.2 / 34.4
29.2 / 32.1
30.4 / 33.6
31.9 / 35.4
NOTES
freedom of the surfactant molecules at the air-liquid
interface compared to that in the cramped interior of
micelle24,25.
The endothermic H0a may be due to the breaking
of H-bonds between polyoxyethylene chain oxygen of
surfactant and water molecules at the air-liquid
interface.
Acknowledgement
One of the authors (RP) is grateful to the UGC,
New Delhi, for financial assistance in the form of a
minor research project, and the Principal, Government
College, Hisar for providing the necessary basic
research facilities.
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